Astronomers get a special star for their Christmas tree

Gamma-ray astronomers get a special star on their
Christmas tree

Roll over, Beethoven - this blast
is for you

Dec.
22, 1999: Astronomy is ending the year with a bang as scientists
across the world take advantage of a unique bit of NASA teamwork
that quickly located a gamma-ray burst, one of the most violent
events in the universe.

As a result, several major observatories, including the Chandra
X-ray Observatory, were able to swing into position within hours
or days of the blast and discover its X-ray, optical and radio
counterparts for the burst. One astronomer has nicknamed the
blast Beethoven because it fell on the anniversary of the composer's
birth (Dec. 16, 1770). Its official name is GRB 991216.

Right: One of the leading theories for the cause of
gamma ray bursts is the collapsar or failed supernova theory.
A super-massive star, after burning all of its nuclear fuel,
starts to explode as a supernova, but the overlying atmosphere
is too massive to blow off, and the explosion collapses, forming
jets of matter that burrow out through the poles and then rip
the star apart. The scale of the events at the core is about
the same size as our Earth. For more details, see the story (link
to Taking a Ringside Seat, below) from the 5th Huntsville Gamma
Ray Burst Symposium. Credit: Stan Woosley, University of California,
Santa Cruz.

"This is the first major success in a three-year program
to use instruments on two NASA satellites to get a rapid location
of a burst," explained Dr. Marc Kippen. Kippen is a University
of Alabama in Huntsville astrophysicist working at NASA's Marshall
Space Flight Center. He's part of the Burst and Transient Source
Experiment (BATSE) team at NASA/Marshall.

BATSE, one of four instruments aboard the Compton Gamma Ray
Observatory, usually is the first (sometimes only) instrument
to detect a gamma ray burst. BATSE's eight detector modules point
from each corner of the Compton Observatory so they will capture
anything that happens above Earth's horizon.

(The Robotic
Optical Transient Search Experiment, or ROTSE, which caught GRB
990123, was in daylight and thus unable to detect this burst.)

The price for this all-sky capability is reduced precision
in determining the location of a burst. With a little processing
on the ground, BATSE data can be used to locate a burst inside
a circle 4 degrees across, about eight times the apparent width
of the Moon. That's still far too wide for most telescopes, whose
high powers also mean very narrow fields of view.

Now the ball is picked up by the Rossi X-ray Timing Explorer,
operated by NASA's Goddard Space Flight Center. Rossi does not
have imaging instruments. Even its sensitive Proportional Counting
Array has a 1 degree field of view, still too large for most
telescopes. But it can be used to produce coarse images.

"What they do is a scan across a region," Kippen
explained. It's a little like the raster scan of an electron
beam across the face of a TV screen: across, down one, back across,
down one, and so on.

"We've done about 25 bursts where we got the location
with BATSE and gave it to Rossi and they spent a frantic time
trying to reprogram the spacecraft to do this scan," he
continued. It can take several hours to reprogram the satellite.
With Beethoven, they got lucky.

"They were able to get this one in four hours,"
Kippen said. "They got the source in the first four scans
and just happened to see the source in two of those scans."

This brought the location down to a narrow box, just 0.04-by-0.3
degree in size. Another scan 10 hours later placed a similar
box at right angles over the location, now pinning it down to
0.04-by-0.08 degrees.

"At
that time, using the Rossi location, fairly large telescopes
started making observations," Kippen said. "The earlier
attempts didn't catch it" when using coarser data. But those
using the refined BATSE and Rossi data did.

Left: The hunt started with a low-intensity trigger,
followed 20 seconds later by an intense blast of gamma radiation.
The trigger event was a flash of radiation just powerful enough
to switch on the BATSE alerting system. In about 20 percent of
bursts, a small precursor burst comes from a few seconds to several
minutes before the main event. No one is yet sure why. Links
to 730x478-pixel,
10KB GIF. Credit: BATSE team, NASA/Marshall Space Flight
Center

The burst is at 77.38 right ascension, 11.30 declination.
The first observatory to catch the optical afterglow was the
MDM - the University of Michigan, Darthmouth College, Massachusetts
Institute of Technology, Columbia University observatory at Kitt
Peak, Ariz. - which quickly recorded a fading magnitude 18.7
source. Since then, several observatories have trained on the
burst and recorded a steadily declining afterglow.

"There is even a radio source coincident with the X-ray
and optical sources," Kippen explained. "Practically
every waveband they've looked at they've seen something."

Even the Chandra X-ray Observatory was able to catch the fading
embers just four days after the event, a remarkable bit of reprogramming
for so complex a facility.

"It is an excellnt choice for Chandra's first GRB observation,"
Kippen said. "It's guaranteed to be well studied."

While detailed understanding will have to await some study,
followed by publication, one early result has been published
in astronomical circulars. An approximation of the red shift
of the blast puts it more than 10 billion light years away, roughly
2 billion years after the Big Bang.

"It's amazing that such a bright burst would have a Z
of 1," Kippen said, referring to the red shift measurement.
A red shift of z=1 indicates that the burst has had its emission
red-shifted by a factor of 2 (1+z) due to the cosmological expansion
of the universe."

The coordinated observations - which ensure a large data set
- alone would be enough to produce a series of professional papers
about GRB 991216. In addition, it ties as the second brightest
burst recorded by BATSE. It didn't just ring the bell; it rang
all eight.

"When you get real high intensities, some photons trickle
through the backside of the detectors," Kippen said. "For
really bright events, it will light up everything."

BATSE's
main large area detectors, or LADs, are large, single-crystal
sheets of sodium iodide. Gamma rays passing through will make
the crystals sparkle or scintillate. This light is picked up
by special sensors at the other end of the bucket holding the
crystal.

Right: Hunting a gamma-ray burst's location is a lot like
detective work. The blue circles represent the area BATSE indicated
the burst should be; the larger the circle, the greater the probability
of including the burst. The diagonal represents the arc of sky
carved out by correlating the different times of arrival as seen
by BATSE and by interplanetary spacecraft with small detectors.
Finally, Rossi searched inside the narrow overlap area (inset)
and quickly found the burst source. Links to 496x382-pixel,
10KB GIF.

Usually, just three or four BATSE detectors see a burst event.
A detector facing the burst will see it the brightest; those
at an angle will see it more dimly. By applying a little trigonometry,
scientists can determine the approximate position in the sky.

In the case of GRB 991216, all eight detectors were triggered
(there are methods of telling which side is illuminated, so there
was no confusion about the direction) because the radiation was
strong enough to go through the spacecraft.

The brightest seen by BATSE, on Sept. 24, 1996 (GRB 960924),
was a mere two times brighter than Beethoven. The previous No.
2 was the "Superbowl burst" that went off Jan. 31,
1993. BATSE scientists were enjoying the game when their pagers
were set off by computers in BATSE control room.

Since then, the alerting system has been refined so notices
are sent automatically, thus speeding the process of putting
observatories on notice, and increasing the chances of catching
the burst in the act with more than BATSE.

1999
GRB Symposium series

Nov 2: Taking a ringside seat
for a gamma-ray burst Supercomputers are giving scientists
a ringside seat for one of the most violent events in nature,
the heart of a gamma ray burst. The "collapsar" model
simulates a star that is too heavy to go supernova, and thus
turns itself inside out.

Oct 29: A Swift Look at the
Biggest Explosions in the Universe Spurred by the thousands
of gamma-ray bursts recorded over the last three decades, NASA
is planning missions dedicated to discovering the causes of what
had been an oddity and now has become a primary mystery.

Oct 25: Postmortems in the Sky
To say they are ghoulish may be going too far, but like ghouls
those studying Gamma Ray Bursts gleefully seek the moldering
remains, and never see the living victim. But they are very much
interested in both the victim and the cause.